This invention relates to a process for treatment and separation of O,O-di-(C.sub.1-8 -alkyl)-phosphorochloridothioates from a mixture thereof with certain impurities. The O,O-dialkyl-phosphorochloridothioates are valuable intermediates, for instance, in the preparation of lubricant additives and insecticides. Particularly, O,O-diethyl thiophosphoryl chloride is an intermediate in the synthesis of an insecticide known generically as parathion and O,O-dimethyl thiophosphoryl chloride is an intermediate for the synthesis of an insecticide generically called methyl parathion. Such are also useful in the manufacture of diazinon, chlorpyrifus, fensulfothion and the like.
Several methods have been taught in the patent literature for the synthesis of the esters of phosphorochloridothioic acid including one-step and two-step methods. In the one-step process, phosphorus pentasulfide, alcohol and chlorine are reacted to prepare the ester corresponding to the alcohol and then the solvent is removed and the product separated.
In the two-step process, the first process step reacts phosphorus pentasulfide with an alcohol, such as ethanol, so as to form O,O-diethyl dithiophosphoric acid and hydrogen sulfide, and in a second process step the isolated O,O-diethyl dithiophosphoric acid is chlorinated in an appropriate solvent with chlorine gas, resulting in the formation of O,O-diethyl thiophosphoric acid chloride. Conventionally, the alkyl groups in the dialkyl phosphorochloridothioates have from 1 to 8 carbon atoms and are generally selected from methyl, ethyl, isopropyl, butyl, sec.-butyl, t-butyl, and the like, up through n-octyl and isomers thereof. However, each of these conventional one-step and two-step processes produces impurities such as phosphates which must be separated from the product. Several solutions to the impurity problem have been proposed in the prior art. In one process, the product is simply distilled under reduced pressure from the reaction vessel. The sump temperature increases to about 150.degree. C. during distillation, and upon cooling after terminating distillation the liquid sump phase solidifies and consists essentially of elementary sulfur (confer U.S. Pat. No. 3,356,774). However, some impurities have such close boiling points that they cannot be separated as produced from the desired O,O-dialkyl phosphorochloridothioate product.
In another prior art patent disclosing a one-step process, the reaction mixture is treated with hydrogen sulfide to convert the sulfur monochloride formed during the reaction of chlorine with the dialkyl dithiophosphoric acid. When the hydrogen sulfide treatment is carried out at relatively low temperature, a precipitate of sulfur is obtained with practically no by-products. Then, by distilling under vacuum and washing the distillate with water, the diesters of phosphorochloridothioic acid are obtained in very high yield and very high degree of purity (U.S. Pat. No. 3,502,750). Unfortunately, such water washing processes require extensive capital investment for equipment and take additional product losses from hydrolysis. In another prior art process using a two-stage chlorination reaction technique, the reaction mixture is chlorinated and then established and maintained at a temperature in the range of 85.degree.-110.degree. C. until it is substantially free of sulfur monochloride and the relatively thermal unstable sulfur that forms becomes more thermally stable so that the product dialkyl thiophosphoryl chloride can be readily and safely removed from the mixture thereof with sulfur by distillation (U.S. Pat. No. 3,836,610).
Further, U.S. Pat. No. 3,897,523 teaches a purification process in which the crude dialkyl phosphorochloridothioate is vaporized in a film evaporator, the vapor is condensed, washed with water at 10.degree. to 60.degree. C., the organic and aqueous phases are separated and the organic phase is vacuum dried. In U.S. Pat. No. 4,025,586, the product dialkyl phosphorochloridothioate is distilled and the distillation residue is water washed to hydrolyze impurities and the washed residue is dried and recycled to the chlorination step.
However, all of the prior art distillation processes do not effect removal of dialkyl phosphates which have similar boiling points to the product dialkyl phosphorochloridothioates of this invention. Known processes which do effect removal of dialkyl phosphates require extensive water washing with increased equipment and its associated capital and operating costs and with additional product losses.
Not only are oxygenated phosphorus impurities, such as alkyl dichlorophosphates and dialkyl chlorophosphates, produced during the reaction as by-products, but the desired dialkyl phosphorochloridothioates may be thermally degraded over time in purification equipment to produce additional by-products of the same sort. The thermal degradation of diethyl phosphorochloridothioate is illustrative of what may occur during processing operations. A sample of diethyl phosphorochloridothioate was found to contain 88.4 area percent diethyl phosphorochloridothioate and 0.48 area percent diethyl chlorophosphate by vapor phase chromatography. The sample was split in two parts and one was purged with air while the other was purged with nitrogen. The samples were heated at 140.degree. C. for 3 hours. Samples taken after one-half hour and 3 hours were analyzed with the results shown below:
______________________________________ Increase of Oxygenated Phosphorus Impurities With Time at 140.degree. C. Time, Hrs. 0 1/2 3 ______________________________________ Air Purge Diethylphosphorochlorido- - thioate (Area %) 88.4 87.3 78.9 Diethylchlorophosphate (Area %) 0.48 0.59 2.4 N.sub.2 Purge Diethylphosphorochlorido- thioate (Area %) 88.4 87.1 71.4 Diethylchlorophosphate (Area %) 0.48 0.68 2.3 ______________________________________
From the above data, it is clear that the impurity has increased fivefold while there was a 9-20% decrease in desired product. Such high impurity levels are undesirable because the end-product insecticides have been registered with governmental agencies as having been tested and found safe and effective with certain impurities at not greater than certain concentrations. Therefore, it is critical to maintain impurity identities and levels at or below those allowed in governmental registrations and as stated on label certifications for the end product.
Without limiting the invention in any manner and without advocating any particular mechanism or theory of action, it is believed that degradation of dialkyl phosphorochloridothioates could possibly take place according to the following chemical reaction scheme: ##STR1## where R.sub.1 and R.sub.2 can be the same or different C.sub.1-8 alkyl groups, the temperature ranges from 100.degree.-150.degree. C. and the necessary contact with ionic species is provided for a time sufficient to facilitate the degradation reaction.
In a related disclosure with a similar purpose, U.S. Pat. No. 3,089,890 teaches treating a distilled crude phosphorochloridothioate with water, separating the organic phase and drying to upgrade the crude and recover substantially contaminant-free phosphorochloridothioate. Most recently, U.S. Pat. No. 4,159,289 teaches a process for removal of sulfur impurities from phosphorochloridothioates by distillation in the presence of a naphthalenic liquid hydrocarbon sulfur solubilizing or suspending agent. However, such distillation would not remove the oxygenated phosphorus impurities made during production of phosphorochloridothioates or from their thermal degradation products. Thus, the need for maintaining and improving the quality of phosphorochloridothioates is readily apparent and required in subsequent processes employing same and in products derived therefrom.